1. Field of the Invention
The present invention relates to a wireless communication device, and in particular to a communication device and method of calibrating frequency offset by awakening the communication device from a sleep mode to a working mode periodically.
2. Description of the Related Art
In a communication system, for example, a TD-SCDMA communication system, a conventional communication device in sleep mode should be awakened into working mode periodically, such as every discontinuous reception (DRX) cycle, to receive page indication channel (PICH) message. However, the conventional communication device is awakened into working mode at the end of each DRX cycle. Since there may be a frequency offset in the clock signal, the communication device maybe unable to be awakened to receive the PICH message within the time range defined in the communication standard. Conventionally, there are two methods to solve the aforementioned problem. In the first method, the communication device may be awakened at the end of each DRX cycle to estimate the accumulated timing offset, and calibrates the frequency offset of the clock signal (e.g. 32 KHz) according to the estimated accumulated timing offset; while in the second method, a high accuracy clock signal (e.g. 26 MHz) could be used to calibrate the clock signal of 32 KHz. However, the aforementioned methods may have some disadvantages. For example, in the first method, when a longer DRX cycle is used or the frequency offset of the 32 KHz clock signal is too large, the accumulated frequency offset may be too large that it exceeds the estimation capability of the conventional communication device, so that the communication device cannot estimate and calibrate the accumulated timing offset properly, resulting in low PICH reception performance; and regarding the second method, an additional calibrating mechanism is required, resulting in higher system complexity. Accordingly, a communication system is demanded to solve the frequency offset calibration issue when the communication system is in the sleep mode (or the idle mode).